A direct current arc furnace requires conduction of the electric arc current through the melt in the hearth of the furnace, in the usual arrangement where the melt has one polarity and the arcing electrode or electrodes have the other polarity. To make the hearth itself electrically conductive for this purpose, is generally considered to be unsatisfactory for various reasons.
Therefore, hearth electrodes of the liquid-solid type have been suggested by the prior art as exemplified by U.S. Pat. No. 3,789,127, dated Jan. 29, 1974, and in a much improved form, by the U.S. Stenkvist U.S. Pat. No. 3,997,712. The electrode may be curved or straight and free from curvature.
The above type is exemplified by a steel billet extending through a furnace construction and having an inside end positioned for contact by a steel melt in the furnace and an outside end provided with cooling means. In operation, the inside end portion melts to the steel's liquid phase, but the steel billet remaining solid from its outer end and safely inwardly towards its inside end, by being cooled, as by water-cooling, the solid outside end being available to form an electrical connection with a direct current power source which is, of course, also connected to an arcing electrode or electrodes.
To accommodate such a hearth electrode, the furnace vessel is constructed with a suitably located passage for insertion of the billet. This passage may be formed by an outwardly extending, possibly removable, extension of the usual steel shell containing the refractory forming the furnace bottom construction.
With the billet positioned in the furnace vessel passage, refractory material in a generally plastic or moldable state, is rammed into the passage or opening around the billet throughout the length of the billet which must extend through the furnace hearth or wall.
The refractory material may, for example, be a magnesite compound made in a plastic or moldable condition by being mixed with water. Therefore, the refractory used to fill the space around the electrode or billet formed by the furnace construction opening, which for practical reasons has a cross sectional area substantially larger than does the billet, is moist in the case of any new electrode installation and requires thorough drying before the furnace can be put into operation.
Heretofore, to dry the rammed and moist refractory around the billet in a new installation, it has been customary to heat the inside end of the billet, not required for an electrical connection with a power line, so that by conduction throughout the billet the necessary drying process is hopefully accelelerated. However, steel has a relatively low thermal conductivity, so the conduction of heat throughout its length is relatively poor, making the drying undesirably slow.
As disclosed in the Fredrikson et al Ser. No. 672,305, filed Mar. 31, 1976, and assigned to the assignee of the present application, a hole is formed throughout the length of the steel billet. Incidentally, the term "steel" is intended to mean not only the plain carbon steel or steel alloys, but iron as well. In other words, the steel billet is internally formed to provide for the conduction of gases throughout the length of the billet. The billet or hearth electrode can be a casting, so any hole formed through it need not necessarily be circular. However, whether formed as a casting or a forged billet which is drilled lengthwise, the construction of such a hearth electrode having an internal lengthwise gas passage, involves undesirable technical problems.
The advantage of such a billet now able to conduct gases, is that the gas flame can be burned at the inside end, free from the billet's outside end where the electric line connection must be made, while at the outside end suction is applied to draw the hot gases thus produced, continuously throughout the length of the billet, thus heating the billet throughout its length.